The invention concerns a power supply and in particular a power supply suitable for DC and pulsed DC plasma processing.
A conventional power supply has the structure shown by FIG. 13 and comprises a H-bridge switching circuit 51, a transformer 52, a rectifier 54 and a filter formed by an inductor 55 and a capacitor 56. FIG. 18 shows a typical output characteristic of such a conventional power supply. As can be appreciated from FIG. 18 such an output characteristic is not suitable for applications in which the electrical load connected to the power supply varies in a broad range, like e.g. in the case of DC plasma processing where the electrical load represented by the plasma does indeed vary in a broad range. FIG. 18 shows that if the electrical load represented by the plasma requires a lower voltage and a higher current, the current capability of a conventional power supply of the type shown by FIG. 13 is relatively limited.
Prior art power supplies for DC plasma processing follow two different approaches in order to enhance the range of DC current delivered to the electrical load represented by the plasma and to obtain an output characteristic of the type represented by FIG. 19.
In a first prior art approach illustrated by FIGS. 14 and 15, the power supply has e.g. the basic structure shown by FIG. 14 which only differs from the conventional power supply shown by FIG. 13 in that a transformer 53 having a primary winding 531 and several secondary windings, e.g. two secondary windings 532, 533, is used instead of transformer 52 in FIG. 13.
Secondary windings 532, 533 can be connected either in parallel for having a low voltage and a high current capability in the case of a DC plasma processing where a low voltage is required, or in series for having a high voltage and a low current capability in the case of a DC plasma processing where a high voltage is required.
In order to increase the voltage range of the power supply shown by FIG. 14 it is necessary to change the connection of the secondary windings 532, 533 from their connection in parallel according to FIG. 14 to their connection in series according to FIG. 15. Manual change of this connection is time consuming and therefore undesirable in an ongoing DC plasma processing.
If the DC plasma processing requires to have two or more plasma types it is necessary to have two or more pre-configured power supplies in order to reduce time loss during the process. The first prior art approach is thus expensive.
In a second prior art approach illustrated by FIGS. 16 and 17, the power supply has a similar structure as in FIGS. 14 and 15, but comprises in addition switches 61, 62, 63 which make it possible to change the connection of the secondary windings 532, 533 from their connection in parallel according to FIG. 16 to their connection in series according to FIG. 17. This second prior art approach has the disadvantage that switches 61, 62, 63 can only be actuated in stand-by mode (not under electrical power) and cannot be actuated during an ongoing plasma processing, because actuation of these switches under electrical power would cause electrical arcs involving the contacts of the switches and would eventually burn these switches. Therefore, this second prior art approach also requires interruption of an ongoing DC plasma processing in order to change the configuration of the connections of the secondary windings 532, 533 in order to modify the output characteristic of the power supply.
A main aim of the instant invention is to provide a power supply and in particular a power supply suitable for DC plasma processing.
A further aim of the invention is to provide a power supply which is able to provide a constant electrical power to an electrical load which varies within a broad range without having to change the hardware configuration of the power supply or to use different arrangements of power supplies for different ranges of electrical power, voltage and current to be applied to such an electrical load.
A further aim of the invention is to provide a power supply which is in particular able to provide a desired constant electrical power for any value of the variable voltage across the electrical load represented by a plasma.
According to the invention the above aim is achieved with a power supply defined by claim 1 or 4. Preferred embodiments of a power supply according to the invention are defined by the dependent claims.
A power supply according to the invention is apt to provide a constant electrical power to an electrical load which for a given voltage varies within a broad range in a ratio of 1 to 10 or more, e.g. the electrical load represented by a plasma. Thus for a given voltage, a power supply according to the invention is apt to satisfy a corresponding variation of the current to be supplied to such an electrical load.
Moreover, a power supply according to the invention is in particular apt to provide a desired constant electrical power for any value of the variable voltage across a plasma.